Altered (copy-up) forms of initiator protein pi suppress the point mutations inactivating the gamma origin of plasmid R6K.

The R6K gamma origin core contains the P2 promoter, whose -10 and -35 hexamers overlap two of the seven binding sites for the R6K-encoded pi protein. Two mutations, P2-201 and P2-203, which lie within the -35 region of P2, are shown to confer a promoter-down phenotype. We demonstrate here that these mutations prevent replication of a gamma origin core plasmid. To determine whether or not the reduced promoter activity caused by these mutations is responsible for their effect on replication, we generated two new mutations (P2-245-6-7 and P2-246) in the -10 hexamer of the P2 promoter. Although these new mutations inhibit P2 activity as much as the P2-201 and P2-203 mutations, they do not prevent replication of the gamma origin core. Therefore, activity of the P2 promoter does not appear to be required for replication. We also show that the inability of the gamma origin to function in the presence of the P2-201 and P2-203 mutations is reversed by the hyperactive variants of pi protein called copy-up pi. This suppression occurs despite the fact that in vivo dimethyl sulfate methylation protection patterns of the gamma origin iterons are identical in cells producing wild-type pi and those producing copy-up pi variants. We discuss how the P2-201 and P2-203 mutations could inhibit replication of the gamma origin core and what mechanisms might allow the copy-up pi mutants to suppress this deficiency.     

Non-essentiality of cysteine and histidine residues for the activity of human class PI glutathione S-transferase.

In order to examine the roles of cysteine and histidine residues in the activity of human class Pi glutathione S-transferase (GST pi), site-directed mutagenesis was used to replace each of the four cysteine residues (at positions 14, 47, 101 and 169) with serine and each of the two histidine residues (at positions 71 and 162) with asparagine using a cDNA for the enzyme (Kano, T. et al. (1987) Cancer Res., 47, 5626-5630) and an E. coli expression system. The replacements of Cys101, Cys169, His71 and His162 did not affect the GSH-conjugating activity toward 1-chloro-2,4-dinitrobenzene and ethacrynic acid. On the other hand, the activities were partly decreased by the replacements of Cys47 and Cys14. These results indicated that the cysteine and histidine residues in GST pi are not essential for the catalytic activity, although Cys47 and Cys14 may contribute to some extent to the catalytic efficiency.     

Histidine Residues in the Na+-coupled Ascorbic Acid Transporter-2 (SVCT2) Are Central Regulators of SVCT2 Function,Modulating pH Sensitivity,Transporter Kinetics,Na+ Cooperativity,Conformational Stability,and Subcellular Localization

Na⁺-coupled ascorbic acid transporter-2 (SVCT2) activity is impaired at acid pH, but little is known about the molecular determinants that define the transporter pH sensitivity. SVCT2 contains six histidine residues in its primary sequence, three of which are exofacial in the transporter secondary structure model. We used site-directed mutagenesis and treatment with diethylpyrocarbonate to identify histidine residues responsible for SVCT2 pH sensitivity. We conclude that five histidine residues, His¹⁰⁹, His²⁰³, His²⁰⁶, His²⁶⁹, and His⁴¹³, are central regulators of SVCT2 function, participating to different degrees in modulating pH sensitivity, transporter kinetics, Na⁺ cooperativity, conformational stability, and subcellular localization. Our results are compatible with a model in which (i) a single exofacial histidine residue, His⁴¹³, localized in the exofacial loop IV that connects transmembrane helices VII-VIII defines the pH sensitivity of SVCT2 through a mechanism involving a marked attenuation of the activation by Na⁺ and loss of Na⁺ cooperativity, which leads to a decreased V_max without altering the transport K_m; (ii) exofacial histidine residues His²⁰³, His²⁰⁶, and His⁴¹³ may be involved in maintaining a functional interaction between exofacial loops II and IV and influence the general folding of the transporter; (iii) histidines 203, 206, 269, and 413 affect the transporter kinetics by modulating the apparent transport K_m; and (iv) histidine 109, localized at the center of transmembrane helix I, might be fundamental for the interaction of SVCT2 with the transported substrate ascorbic acid. Thus, histidine residues are central regulators of SVCT2 function.     

Raman spectroscopic study on the copper(II) binding mode of prion octapeptide and its pH dependence.

The cellular form of prion protein is a precursor of the infectious isoform, which causes fatal neurodegenerative diseases through intermolecular association. One of the characteristics of the prion protein is a high affinity for Cu(II) ions. The site of Cu(II) binding is considered to be the N-terminal region, where the octapeptide sequence PHGGGWGQ repeats 4 times in tandem. We have examined the Cu(II) binding mode of the octapeptide motif and its pH dependence by Raman and absorption spectroscopy. At neutral and basic pH, the single octapeptide PHGGGWGQ forms a 1:1 complex with Cu(II) by coordinating via the imidazole N pi atom of histidine together with two deprotonated main-chain amide nitrogens in the triglycine segment. A similar 1:1 complex is formed by each octapeptide unit in (PHGGGWGQ)2 and (PHGGGWGQ)4. Under weakly acidic conditions (pH approximately 6), however, the Cu(II)-amide- linkages are broken and the metal binding site of histidine switches from N pi to N tau to share a Cu(II) ion between two histidine residues of different peptide chains. The drastic change of the Cu(II) binding mode on going from neutral to weakly acidic conditions suggests that the micro-environmental pH in the brain cell regulates the Cu(II) affinity of the prion protein, which is supposed to undergo pH changes in the pathway from the cell surface to endosomes. The intermolecular His(N tau)-Cu(II)-His(N tau) bridge may be related to the aggregation of prion protein in the pathogenic form.     

The role of histidine in the anemia of folate deficiency

The amino acid histidine is metabolized to glutamic acid in mammalian tissue. Formiminoglutamic acid (FIGLU) is an intermediary in this reaction, and tetrahydrofolic acid is the coenzyme that converts it to glutamic acid. A test for folate deficiency concerns the measurement of urinary FIGLU excretion after a histidine load. It was observed that folate-deficient individuals receiving the histidine for the FIGLU test made hematological response that alleviated the anemia associated with this deficiency. This was unusual in that a biochemical test to determine the deficiency results in a beneficial effect for one aspect of the deficiency. The studies reported in this paper give a metabolic explanation for this phenomenon. Urine was collected for 24 hr from 25 folate-deficient subjects, 10 vitamin B(12)-deficient subjects, and 15 normal controls. Urinary excretion of histidine was a mean of 203 mg with a range of 130-360 mg for the folate-deficient subjects; 51.5 mg with a range of 30-76.6 mg for normal subjects; and 60.0 mg with a range of 32.3-93.0 mg for the vitamin B(12)-deficient subjects. All the folate-deficient subjects subsequently made a hematological response to the histidine administered for the FIGLU test. No hematological response was observed in the vitamin B(12)-deficient individuals. When folic acid was given to folate-deficient subjects who received no histidine, urinary histidine levels returned to normal levels rapidly and this was followed by a hematological response. Others have shown that volunteers fed a histidine-free diet developed anemia. In normal subjects, histidine is excreted much more in the urine than other essential amino acids are. Hemoglobin protein contains 10% histidine. Under normal conditions, dietary histidine can supply sufficient histidine to prevent anemia. When the dietary intake is diminished or the urinary excretion is greatly increased, anemia results. It is concluded that folate deficiency causes histidine depletion through increased urinary excretion of this amino acid. Feeding histidine replenishes tissue levels of histidine, resulting in hemoglobin regeneration. Folic acid administration results in return of histidine to normal urinary levels. Thus, a combination of folic acid histidine would be beneficial for folate deficient individuals.     

Identification of the titrating group in the heme cavity of myoglobin. Evidence for the heme-protein pi-pi interaction

The pH dependence of the proton NMR chemical shifts of met-cyano and deoxy forms of native and reconstituted myoglobins reflects a structural transition in the heme pocket modulated by a single proton with pK 5.1-5.6. Comparison of this pH dependence of sperm whale and elephant myoglobin and that of the former protein reconstituted with esterified hemin eliminates both the distal histidine as well as the heme propionates as the titrating residue. Reconstitution of sperm whale met-cyano myoglobin with hemin modified at the 2,4-positions leads to a systematic variation in the pK for the structural transition, thus indicating the presence of a coupling between the titrating group and the heme pi system. The results are consistent with histidine FG3 (His-FG3) being the titrating group, and a donor-acceptor pi-pi interaction between its imidazole and the heme is proposed.     

Heteronuclear 2D (1H-13C) MAS NMR Resolves the Electronic Structure of Coordinated Histidines in Light-Harvesting Complex II: Assessment of Charge Transfer and Electronic Delocalization Effect

In a recent MAS NMR study, two types of histidine residues in the light-harvesting complex II (LH2) of Rhodopseudomonas acidophila were resolved: Type 1 (neutral) and Type 2 (positively charged) (Alia et al. J. Am. Chem. Soc. ). The isotropic (13)C shifts of histidines coordinating to B850 BChl a are similar to fully positively charged histidine, while the (15)N shift anisotropy shows a predominantly neutral character. In addition the possibility that the ring currents are quenched by overlap in the superstructure of the complete ring of 18 B850 molecules in the LH2 complex could not be excluded. In the present work, by using two-dimensional heteronuclear ((1)H-(13)C) dipolar correlation spectroscopy with phase-modulated Lee-Goldburg homonuclear (1)H decoupling applied during the t(1) period, a clear and unambiguous assignment of the protons of histidine interacting with the magnesium of a BChl a molecule is obtained and a significant ring current effect from B850 on the coordinating histidine is resolved. Using the ring current shift on (1)H, we refine the (13)C chemical shift assignment of the coordinating histidine and clearly distinguish the electronic structure of coordinating histidines from that of fully positively charged histidine. The DFT calculations corroborate that the coordinating histidines carry approximately 0.2 electronic equivalent of positive charge in LH2. In addition, the data indicate that the ground state electronic structures of individual BChl a /His complexes is largely independent of supermolecular pi interactions in the assembly of 18 B850 ring in LH2.     

Identification of Des-methyl-DIF-1 Methyltransferase in Dictyostelium purpureum

For the production of D-amino acids using stable N-carbamyl-D-amino acid amidohydrolase (DCase) in an immobilized form, the DCase gene of Agrobacterium sp. KNK712 was mutagenized to increase its enzymatic thermostability. In a search for thermostability-related amino acid sites besides the two known sites of DCase, i.e., the 57th and 203rd amino acids, the new mutant enzyme found, in which the 236th amino acid, valine, had been changed to alanine, showed a 10°C increase in thermostability. These known three thermostability-related amino acids were changed to other amino acids by the PCR technique, and it was proved that the thermostability of the DCase increased when the 57th amino acid of DCase, histidine, was changed to leucine, the 203rd amino acid, proline, to asparagine, glutamate, alanine, isoleucine, histidine, or threonine, and the 236th amino acid, valine, to threonine or serine, in addition to the known mutations.     

Inhibitory effect of copper(II) on zinc(II)-induced aggregation of amyloid beta-peptide

Aggregation of amyloid beta-peptide (Abeta), a key pathological event in Alzheimer's disease, has been shown in vitro to be profoundly promoted by Zn(II). This fact suggests that some factors in the normal brain protect Abeta from the Zn(II)-induced aggregation. We demonstrate for the first time that Cu(II) effectively inhibits the Abeta aggregation by competing with Zn(II) for histidine residues. The Raman spectrum of a metal-Abeta complex in the presence of both Zn(II) and Cu(II) shows that the cross-linking of Abeta through binding of Zn(II) to the N(tau) atom of histidine is prevented by chelation of Cu(II) by the N(pi) atom of histidine and nearby amide nitrogens. The inhibitory effect is strongest at a Cu/Abeta molar ratio of around 4. Above this ratio, Cu(II) itself promotes the Abeta aggregation by binding to the phenolate oxygen of Tyr10. These results emphasize the importance of regulation of Cu(II) levels to inhibit Abeta aggregation, and are consistent with an altered metal homeostasis in Alzheimer's disease.     

The histidine-rich glycoprotein of serum has a domain rich in histidine, proline, and glycine that binds heme and metals

Histidine-rich glycoprotein (HRG) from rabbit serum was digested with plasmin, reduced, and carboxymethylated, and the fragments produced were resolved by reverse-phase high-performance liquid chromatography. Several peptide fractions were obtained that contain unusually high contents of histidine, proline, and glycine. One His-Pro-Gly-rich peptide (apparent Mr 30 000) was obtained in sufficient yield and purity for further study. This peptide is 29 mol % histidine, 37% proline, and 16% glycine, indicating that most of these three amino acids are located in one region of HRG. The peptide contains 9% by weight carbohydrate and is devoid of tyrosine or tryptophan. The far-ultraviolet circular dichroism spectrum of the peptide has a minimum at 203 nm, indicating that the peptide contains polyproline II helical sections. The peptide represents a binding domain of HRG since it retains much of the ability of intact HRG to bind heme and metals including Zn2+, Ni2+, and Cu2+. As with the parent HRG molecule, interaction of the peptide with heme and metals is dependent on pH and intact histidine residues.     

Engineering a beta-carotene ketolase for astaxanthin production

A new beta-carotene ketolase gene (crtW) was cloned from an environmental isolate Sphingomonas sp. DC18. A robust and reliable color screen was developed for protein engineering to improve its activity on hydroxylated carotenoids for astaxanthin production. Localized random mutagenesis was performed on the crtW gene including the upstream ribosomal binding site (RBS). Six mutations (H96L, R203W, A205V, A208V, F213L and A215T) in the crtW gene were isolated multiple times that showed improved astaxanthin production. These mutations were localized near the conserved histidine motifs, which were proposed for binding iron required for enzymatic activity. Combination of two of the mutations (R203W/F213L) further improved astaxanthin production. One mutation at the RBS (a438t) was shown to have additional effect on improving astaxanthin production. Most of the mutants still retained high activity on beta-carotene, however, the F213L single mutant and the R203W/F213L double mutant that yielded the highest improvement for astaxanthin production showed decreased activity for canthaxanthin production.     

Primary and secondary structural analyses of glutathione S-transferase pi from human placenta

The primary structure of glutathione S-transferase (GST) pi from a single human placenta was determined. The structure was established by chemical characterization of tryptic and cyanogen bromide peptides as well as automated sequence analysis of the intact enzyme. The structural analysis indicated that the protein is comprised of 209 amino acid residues and gave no evidence of post-translational modifications. The amino acid sequence differed from that of the deduced amino acid sequence determined by nucleotide sequence analysis of a cDNA clone (Kano, T., Sakai, M., and Muramatsu, M., 1987, Cancer Res. 47, 5626-5630) at position 104 which contained both valine and isoleucine whereas the deduced sequence from nucleotide sequence analysis identified only isoleucine at this position. These results demonstrated that in the one individual placenta studied at least two GST pi genes are coexpressed, probably as a result of allelomorphism. Computer assisted consensus sequence evaluation identified a hydrophobic region in GST pi (residues 155-181) that was predicted to be either a buried transmembrane helical region or a signal sequence region. The significance of this hydrophobic region was interpreted in relation to the mode of action of the enzyme especially in regard to the potential involvement of a histidine in the active site mechanism. A comparison of the chemical similarity of five known human GST complete enzyme structures, one of pi, one of mu, two of alpha, and one microsomal, gave evidence that all five enzymes have evolved by a divergent evolutionary process after gene duplication, with the microsomal enzyme representing the most divergent form.     

Metal binding modes of Alzheimer's amyloid beta-peptide in insoluble aggregates and soluble complexes

Aggregation of the amyloid beta-peptide (Abeta) into insoluble fibrils is a key pathological event in Alzheimer's disease. Zn(II) induces the Abeta aggregation at acidic-to-neutral pH, while Cu(II) is an effective inducer only at mildly acidic pH. We have examined Zn(II) and Cu(II) binding modes of Abeta and their pH dependence by Raman spectroscopy. The Raman spectra clearly demonstrate that three histidine residues in the N-terminal hydrophilic region provide primary metal binding sites and the solubility of the metal-Abeta complex is correlated with the metal binding mode. Zn(II) binds to the N(tau) atom of the histidine imidazole ring and the peptide aggregates through intermolecular His(N(tau))-Zn(II)-His(N(tau)) bridges. The N(tau)-metal ligation also occurs in Cu(II)-induced Abeta aggregation at mildly acidic pH. At neutral pH, however, Cu(II) binds to N(pi), the other nitrogen of the histidine imidazole ring, and to deprotonated amide nitrogens of the peptide main chain. The chelation of Cu(II) by histidine and main-chain amide groups results in soluble Cu(II)-Abeta complexes. Under normal physiological conditions, Cu(II) is expected to protect Abeta against Zn(II)-induced aggregation by competing with Zn(II) for histidine residues of Abeta.     

Primary structure of yeast cytochrome c peroxidase. II. The complete amino acid sequence

The complete amino acid sequence of 293 residues in the single polypeptide chain of yeast cytochrome c peroxidase has been determined. Sequence analyses were performed on fragments obtained by cleavage with cyanogen bromide and by tryptic digestion of citraconylated protein. These fragments were aligned with sequential and compositional data as well as those obtained with intact protein, tryptic, and chymotryptic peptides (Takio and Yonetani, 1980, Arch. Biochem. Biophys.203, 605–614). Residues critical for catalysis by the enzyme were identified as arginine 48, tryptophan 51, histidine 52, and histidine 174 by fitting the sequence to the electron density map derived by others. Sequence comparison with other proteins show limited homology with horseradish peroxidase and myoglobin.     

Resonance Raman characterization of the mononuclear iron active-site vibrations and putative electron transport pathways in Pyrococcus furiosus superoxide reductase

The resonance Raman spectrum of oxidized wild-type P. furiosus SOR at pH 7.5 and 10.5 has been investigated using excitation wavelengths between 406 and 676 nm, and vibrational modes have been assigned on the basis of isotope shifts resulting from global replacements of (32)S with (34)S, (14)N with (15)N, (56)Fe with (54)Fe, and exchange into a H(2)(18)O buffer. The results are interpreted in terms of the crystallographically defined active-site structure involving a six-coordinate mononuclear Fe center with four equatorial histidine ligands and axial cysteine and monodentate glutamate ligands (Yeh, A. P., Hu, Y., Jenney, F. E., Adams, M. W. W., and Rees, D. C. (2000) Biochemistry 39, 2499-2508). Excitation into the intense (Cys)S(p(pi))-to-Fe(d(pi)) CT transition centered at 660 nm results in strong enhancement of modes at 298 cm(-1) and 323 cm(-1) that are assigned to extensively mixed cysteine S-C(beta)-C(alpha) bending and Fe-S(Cys) stretching modes, respectively. All other higher-energy vibrational modes are readily assigned to overtone or combination bands or to fundamentals corresponding to internal modes of the ligated cysteine. Weak enhancement of Fe-N(His) stretching modes is observed in the 200-250 cm(-1) region. The enhancement of internal cysteine modes and Fe-N(His) stretching modes are a consequence of a near-planar Fe-S-C(beta)-C(alpha)-N unit for the coordinated cysteine and significant (His)N(p(pi))-Fe(d(xy))-(Cys)S(p(pi)) orbital overlap, respectively, and have close parallels to type 1 copper proteins. By analogy with type 1 copper proteins, putative superexchange electron-transfer pathways to the mononuclear Fe active site are identified involving either the tyrosine and cysteine residues or the solvent-exposed deltaN histidine residue in a Y-C-X-X-H arrangement. Studies of wild-type at pH 10.5 and the E14A variant indicate that the resonance Raman spectrum is remarkably insensitive to changes in the ligand trans to cysteine and hence are inconclusive concerning the origin of the alkaline transition and the nature of sixth Fe ligand in the E14A variant.     

Role of arginine 220 in the oxygen sensor FixL from Bradyrhizobium japonicum

In the heme-based oxygen sensor protein FixL, conformational changes induced by oxygen binding to the heme sensor domain regulate the activity of a neighboring histidine kinase, eventually restricting expression of specific genes to hypoxic conditions. The conserved arginine 220 residue is suggested to play a key role in the signal transduction mechanism. To obtain detailed insights into the role of this residue, we replaced Arg(220) by histidine (R220H), glutamine (R220Q), glutamate (R220E), and isoleucine (R220I) in the heme domain FixLH from Bradyrhizobium japonicum. These mutations resulted in dramatic changes in the O(2) affinity with K(d) values in the order R220I < R220Q < wild type < R220H. For the R220H and R220Q mutants, residue 220 interacts with the bound O(2) or CO ligands, as seen by resonance Raman spectroscopy. For the oxy-adducts, this H-bond modifies the pi acidity of the O(2) ligand, and its strength is correlated with the back-bonding-sensitive nu(4) frequency, the k(off) value for O(2) dissociation, and heme core-size conformational changes. This effect is especially strong for the wild-type protein where Arg(220) is, in addition, positively charged. These observations strongly suggest that neither strong ligand fixation nor the displacement of residue 220 into the heme distal pocket are solely responsible for the reported heme conformational changes associated with kinase activity regulation, but that a significant decrease of the heme pi(*) electron density because of strong back-bonding toward the oxygen ligand also plays a key role.     

The structural bases for the unique ligand binding properties of Glycera dibranchiata hemoglobins. A resonance Raman study

The hemoglobin of the marine annelid Glycera dibranchiata possesses several unique features: the hemoglobin consists of multiple monomeric and polymeric components, quaternary structure is lacking, the distal histidine is replaced by leucine in at least one monomeric constituent, and 4) the protein exhibits extremely rapid ligand binding kinetics. The effect of these structural modifications on the ligand binding process has been evaluated using resonance Raman spectroscopy to examine the vibrational modes of the porphyrin macrocycle in deoxy and carbonmonoxy equilibrium species of hemoglobin G. dibranchiata in both the unseparated monomeric and polymeric forms and in a single monomeric component designated Fraction II. Significant differences relative to hemoglobin were found in porphyrin pi electron density, vinyl environment, low frequency vibrational modes, and, in particular, the Fe-proximal histidine stretching mode. Spectra of the deoxy heme transients generated within 10 ns of ligand photolysis have also been examined. These clearly indicate large differences in the heme pocket dynamics subsequent to CO photolysis in G. dibranchiata hemoglobins relative to other hemoglobins. The significance of these results in terms of the kinetics and thermodynamics of ligand binding is discussed.     

Class pi glutathione S-transferase from pig lung. Purification, biochemical characterization, primary structure and crystallization

A cytosolic glutathione S-transferase from pig lung was purified 210-fold to apparent homogeneity. The enzyme was classified as a class pi isoenzyme on the basis of its physical and chemical properties. It is homodimeric with a subunit Mr of 23,500, has a pI of 7.2, and shows a high specific activity towards ethacrynic acid. The glutathione analogues, S-hexylglutathione and glutathione sulfonate, were strong reversible inhibitors. The enzyme's primary structure, established entirely by protein chemical methods, consists of 203 amino acids and is highly similar (82-84% residue identity) to the rat and human class pi isoenzymes. Furthermore, there was no evidence of microheterogeneity or post-translational modifications. Each subunit contains a highly reactive cysteine residue, the modification of which leads to enzyme inactivation. None of the cysteine residues in the pig enzyme appear to form intramolecular disulfide bonds. Singel crystals of the glutathione-S-transferase-glutathione-sulfonate complex were obtained by the hanging-drop method of vapour diffusion from poly(ethylene glycol) 4000 solutions. The crystals belong to the orthorhombic space group P212121 with unit cell dimensions of a = 10.125 nm, b = 8.253 nm and c = 5.428 nm and diffract to better than 0.22 nm.     

Critical amino acids for the 8(R)-dioxygenase activity of linoleate diol synthase. A comparison with cyclooxygenases

7,8-Linoleate diol synthase (7,8-LDS) of the take-all fungus and cyclooxygenases can be aligned with approximately 24% amino acid identity and both form a tyrosyl radical during catalysis. 7,8-LDS was expressed in insect cells with native 8R-dioxygenase and hydroperoxide isomerase activities. We studied conserved residues of 7,8-LDS, which participate in cyclooxygenases for heme binding (His residues), hydrogen abstraction (Tyr), positioning (Tyr, Trp), and ionic binding of substrates (Arg). Site-directed mutagenesis abolished 8R-dioxygenase activities with exception of the putative distal histidine (His203Gln) and a tyrosine residue important for hydrogen bonding and substrate positioning (Tyr329Phe). The results demonstrate structural similarities between 7,8-LDS and cyclooxygenases.